Tonga, who scored eight tries in a dozen games for the Kangaroos, arrives after a season with Catalans Dragons in Super League.
The former Queensland State of Origin star, 32, is the latest big-name recruit for a club that finished top of the Championship last season but then missed out on promotion after failing miserably in the Super 8s.
Ex-England stand-off Rangi Chase, Samoa captain Harrison Hansen and former Hull KR second-row Corey Paterson have all joined from Salford Red Devils in readiness for 2016 along with another international player Reni Maitua.
